1. Field of the Invention
This invention relates to an automatic flash mechanism (hereinafter called the FA mechanism) for a camera having a focal length varying device arranged to permit selection of one of at least two different focal lengths by shifting an optical system.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings shows by way of example the optical system of a camera having a focal length varying device. The photographic optical system of the camera consists of a main lens system A and an auxiliary lens system B which is arranged to be movable to and away from the optical axis of the main lens system A. Normal photography is performed solely with the main lens system A by retracting the auxiliary lens system B out of the path of the optical axis as shown in FIG. 1(a). The camera permits tele-photography. In that event, the main lens system A is moved forward while the auxiliary lens system B is positioned behind the former on the optical axis to perform photography with the whole optical system as shown in FIG. 1(b). In FIG. 1, reference symbols C and D respectively denote a stop and a film surface. Various kinds of variable focal length cameras have been known, for example, from U.S. Pat. No. 3,388,650, Japanese Laid-Open Patent Applications No. SHO 52-76919 and 54-33027, etc. These cameras of the prior art include many cameras that are arranged to use a stop device in common for the optical system before focal length change-over and the optical system after the change-over as represented by the stop C of FIG. 1. In the camera of this type, however, the F-number of the optical system changes due to change-over between one focal length to another after shifting of the optical system. Therefore, in case of flash photography, the aperture of the stop must be varied, for the same object distance, according to the focal length change-over. This requires provision of some adjustment mechanism for varying the aperture. In many cases, however, such adjustment mechanism is omitted to avoid complication of structural arrangement. Such cameras are incapable of performing an accurate exposure and thus have to rely upon the latitude of the film.
To solve this problem, it has been proposed to incorporate the guide number of the flash device in the camera variable in association with the focal length change-over of the optical system. This method is carried out by effecting a change in flash light emission quantity by changing the charge capacity of the flashing capacitor or by arranging a light reducing plate in front of the flash device. The method, therefore, requires some additional space within the camera for arranging a plurality of capacitors there or it wastes the flash light quantity.